Process of simultaneously wet grinding and classifying a sulfide ore



y 7, 1957 I J. F. MYERS 2,791,382

PROCESS OF SIMULTANEOUSLY WET GRINDING AND CLASSIFYING A SULFIDE ORE Filed Oct. 30, 1953 POOL ROTATION INVEN TOR.

{ John F. Myers ,wujz au ATTQR NEYS 2,791,382 C Patented May 7, 1957 PRSCESS F SIMULTANEGUSLY WET GRINDING AND CLASSHFYING A SULFIDE ORE John F. Myers, Greenwich, Conn.

Appiication October 30, 1953, Serial No. 389,273

10 1 Claim. (Cl. 241-24) This invention relates to improvements in the grinding and classification of minerals and like solids, in particular, the grinding and classification of metallic and nonmetallic ores prior to beneficiation by flotation or other procedures for the separation of the values of the ore from the gangue.

A wide range of ores are treated to concentrate the values and eliminate the gangue by flotation or other procedures which require as a preliminary step the grinding of the ore to a sufficiently finely divided state so that the values are severed from the gangue and the product is susceptible to the beneficiation treatment. As a general rule it may be said that for flotation the ore is ground to minus mesh, that is, to a state of subdivision such that it contains few if any particles which will not pass through a 35 mesh screen, and for a number of products even finer grinding, e. g., with a 48 mesh, 65 mesh or even 100 mesh screen as the limiting upper size is common. in general, such comminution of the ore is obtained by a combination of grinding and classifying steps, with the last grinding step being carried out in a ball mill, using a suspension of the ore in water (pulp), with the efiiuent from the ball mill going to an external classifier wherein oversized material is separated and returned to the ball mill. In the case of a number of ores and mineral articles, material which is too finely divided for treatment (slirnes) is also separated and discarded. This constitutes a substantial loss.

With sulfide ores, e. g., copper, lead and zinc sulfide ores Which are normally concentrated by flotation, the time element involved in the wet grinding and classification is important because of progressive oxidation which reduces recovery and increases the amount of reagent, in particular, collector, required in the flotation system. In addition, the lesser the amount of ultrafine slime (material of size less than 1600 mesh) the more effective is the recovery of values in the flotation operation, as recovery is most effective with fine sands or 100- to MUG-mesh), as may be seen from the tailing analyses in Table V below. With some iron ores and other non-sulfide crudes, it is necessary to discard a slime fraction from the ground material, as the very finely divided values are not only not effectively sep arable from very finely divided gangue, but also the slime interferes with separation among the sands.

Heretofore in the wet ball mill grinding of ores carried on as a preliminary step to flotation or other separation, involving wet grinding in a ball mill followed by external classification, the distribution of particle size in the ground product has been substantially beyond control. Once a fixed upper limit on the size of the particles, dictated by the requirements of the flotation or other operation, is decided, the amount of slime produced is more or less fixed. In'addition, the grinding of sulfide ores in this manner, the time period of exposure of the freshly exposed sulfide surfaces (exposed upon comminution) to aqueous solution prior to flotation has necessarily been prolonged to include the further time required in the classification operation. The latter is the more serious because the time required in the classification operation may be several times as long as in the mill, fresh oxygenated water is added in the classification operation, and the competition for oxygen in the classification operation is much less than in the grinding mill wherein a large area of fresh rustable surface is continually renewed.

The present invention provides improvements in the wet grinding of ores which, with grinding to any given maximum size, in particular to maximum sizes of 35 mesh or less, (1) reduce by 50 to percent the time of exposure of the ore to oxidation, (2) eliminate the necessity for installation of conventional external classifying equipment with concomitant addition of fresh oxygenated water, (3) provide a larger proportion of the ground material in the larger and fine intermediate mesh sizes (sands 1600 mesh) as distinguished from ultrafine 1600 mesh), and (4) increase the capacity and effectiveness of subsequent conventional concentrating equipment.

The improved results of the present invention, in respect of wet ball mill grinding of ores, depend upon a discovery which I have made, which is that if a ball mill is operated with in-mill classification, with a pulp density not greater than that appropriate for conventional mechanical classification of the ore being ground, and with such slow speedof rotation of the mill and such size of ball relative to the mill diameter that cataracting of the balls does not take place, and with a ball charge such as to provide a quiet pool over the submerged portion of the ball charge greater than about 10 inches in depth over the toe and advantageously 20 inches or more in depth, the efiluent from the mill will be a classified product regulable to the desired maximum mesh size by control of the pulp density, with a lower content of slime than the ground product resulting from conventional grinding followed by classification. The exposure of the values liberated by grinding to oxidative influences is minimized, subsequent concentration by all known meth ods is improved, and the requirements of reagent in flotation are materially reduced. The reason for the marked improvement in operating characteristics is not clear to me. It involves change from conventional operating procedure of ball mills by a substantial reduction in pulp density, a change contrary to what those skilled in the art have heretofore thought of as appropriate, in that it has heretofore been thought that reduction in pulp density reduces ball mill efficiency in wet grinding, and the effort has always been made to use a maximum pulp density, using the minimum amount of water which will give a pulp of a consistency which will pass through the ball mill in a time period appropriate for grinding.

The pulp density maintained in the mill in accordance with the present invention will vary from ore to ore and with the limiting mesh desired but will have, as its general range, the range appropriate for classification of the same ore to the same limiting mesh in conventional external classification equipment, such as a reciprocating-rake or spiral classifier, or a hydroscillator. For massive pyritic copper ore, such as that found at Copperhiil in Tennessee, the maximum pulp density may be about 35 to 38 percent by weight for a 35mesh grind, and, in general, this is about the maximum for any ore. With some ores, such as those which are clayey in character or low in heavy mineral content, the maximum pulp densities may be considerably less than this, but in any event are readily determinable from the literature dealing with classification of ground ores. See, for example, Taggart, Handbook of Mineral Dressing.

The equipment used in the practice of the invention is nun...

substantially conventional, that is, what is used is a conventional ball mill adapted for the wet grinding of ores, using spherical balls. The mill is operated at a relatively slow speed (about 55 percent of critical speed), the maximum speed being such that the balls do not cataract and disturb the pool by free fall thereinto, but slid-e or roll down the relatively steep slope above pool level on the high side and thence travel at a relatively low slope across to the toe. The balls circle under at the toe and start across and upward again. Thus the grinding action does not involve impact by free falling balls but either rolling or sliding or both.

The maximum size of ball that can be used in a given mill run at a given speed without cataracting is not subject to precise calculation so far as I know. I do know, however, that three-inch balls cataract in a -foot mill run at 13.8 R. P. M., wherefore the diametral ratio of ball to mill should be smaller than 1:40 under those conditions. The results cited herein were obtained using one-inch balls in a 10-foot mill or a diametral ratio of 1:120 and subsequent operations were at a ratio of 1:104 with no cataracting. Liner conformation affects the ratio. Liner ribs preferably should not exceed the ball diameter or if they do, they should slope backwardly sufficiently on the leading edge to prevent pocketing that might bring about cataracting. In view of the great variety of liner shapes commonly used, if for any reason it is desired to use balls as large as possible one should start with an original charge at a diametral ratio of 1:120 and then use successively larger balls for make-up until the non-cataracting limit is reached. It may be noted here that with 10- or 14-1nesh material as feed to the ball mill when making a 35- or finer mesh final product, which is the range for which in-mill classification is best suited, grinding efiiciency will probably be better with a 1:120 diametral ratio than with a larger one, e. g. 1:50.

The ball load should be of such volume as a maximum that the surface of the load does not disturb the pool at the discharge outlet. I have operated a 10-foot mill with 33-inch discharge trunnion at 13.7 R. P. M. with a 45- percent ball load without such disturbance and under those circumstances the face of the upraised ball mass was about 6 inche from the nearest point of the trunnion opening. If it is desirable to carry a larger load the trunnion may be extended inwardly a short distance and a stationary splash board, externally supported, slipped behind the extension.

The maximum rate of feed to the mill when making a product of a given limiting size will equal and in all probability will exceed the maximum capacity of the same mill operating conventionally at the same speed with an external classifier. This is because the product with in-mill classification contains less 200-1nesh material and this material requires more work to produce it than do the coarser sand sizes. Hence with equal energy available in the ball load in both methods of operation of the same mill, maximum utilization of that energy would enable more tonnage to be ground to limiting size in the mill operating with in-mill classification.

As a consequence of the proper correlation of speed of rotation of the mill, ball load, ratio of ball diameter to mill diameter, and pulp density, there is maintained above the submerged part of the tumbling ball mass within the mill a pool, deepest where the mass of balls touches the downwardly moving part of the shell (the toe) and shallowest on the rising side of the axial line of the mill. The pool extends from, at or near the feed end to the dis charge end, is relatively quiescent on top, but is agitated at the bottom both by the balls moving across underneath and by jets of pulp expelled into the bottom from the underlying ball mass as a consequence of the hydraulic pressure resulting from the lifting of pulp along with balls at the upwardly moving side of the mill. This bottom agitation serves to produce and maintain a teeter column of the larger and heavier grains in the bottom of the pool.

This mass of teetering grains, relatively free from disturbance as compared with similar teeter columns in conventional classifiers of the reciprocating-rake or spiral types, is part of the reason that the pool serves as a surprisingly effective classifying pool, with the fines concentrating toward the top of it and the coarser material which requires further grinding settling through the teeter column at the bottom and into the region of grinding, while the longitudinal flow of pulp through the mill carries the upper stratum of the pool to the discharge at an accelerating velocity characteristic of the approach to a constricted weir. A classified pulp discharges with a minimum concentration of slimes as compared with the concentration of slimes in ground ores produced by conventional grinding and classification and with less preferential grinding of sulfides than occurs in conventional grinding circuits.

Another factor that undoubtedly contributes to the effective pool action is upwardly directed bottom feeding of the pulp to be classified in the form of a multitude of minute pulp streams pouring into the pool all over the bottoms from the underlying ball mass. In all other mechanical classifiers the pulp is introduced either as a sheet spilled onto the pulp surface, or in a plunging single stream either onto or beneath the pulp surface, or by a pipe led into the side of the pool. All of these methods tend to serious local disturbance of the pool and teeter column and in most cases the disturbance spreads considerable distances from the point of pulp entry. None of the methods gives the entering particles initial upward momentum as my method of feeding does and I attribute much of the preferential sand-over-slime production that I achieve to this difference.

Still another advantage of in-mill classification lies in the method of returning insufficiently ground material to the ball load. In conventional mill operation with external classifiers such material is dumped into the feed end of the mill together with new feed with the result that a considerable part of the mill length at this end is chronically overloaded and ineffective. In my method the return sands are spread onto literally millions of points on the surface of the load underneath the pool and are immediately engulfed into a fully active grinding region. Feed-end overloading is at the same time reduced to that caused by the new feed itself which in conventional practice usually amounts to about 20 or 30 percent of the total overloading and rarely exceeds percent.

Thus my method of operating a conventional ball mill with in-mill classification embodies both a new kind of mechanical classifier with novel upward feed of pulp more or less uniformly distributed in small streams all over the bottom of the classifying pool, with uniform gentle agitation of the bottom of the pool, with rapid skimming of the finished material from the surface of the pool, and with discharge of unfinished material directly back to the grinding zone without exposure to increase in oxidizing conditions, and a new method of introducing'return sands from the classifier to the grinding zone in such a way as to distribute it rapidly throughout the ball load and not to aggravate overloading at the feed end.

I am aware that rudimentary differential sedimentation occurs in the pulp above the balls in ball mills operating conventionally just as it does in any semi-fluid mixture of any finely divided solid and an agitated liquid. In face I published such an observation in Trans. AIME, vol. 187, p. 710, in June, 1950. But these bodies of pulp in conventionally operated mills are too dense and normally too turbulent to effect the close size cut required for preparation of feed for fine-particle separators. Thus in the publication cited, describing results in a relatively undisturbed pool, but of conventional densities (47.0 and 62.3 percent solids), the surface pulp contained 28- mesh material and too little 200-mesh material for satisfactory' flotation, and had to be finished in an external classifier. By going counter to the accepted ball mill 7 practice of maintaining as thick a pulp in the mill as possible without causing cushioning and diluting the pulp in the mill to the consistency of the normal overflow of an external classifier operating on the same ore, I was able to super-impose on the normal grinding function of the mill a classifying function more efiicient than that of the conventional external classifier, and do' so with no loss in grinding capacity. I am also aware that ultrafine wet grinding can be and is elfected in singlepass open-circuit tube mills (operating without classifiers), whose length is several times their diameter, but this is done at great and unnecessary expense of power, in thick pulps, by reducing the normal per foot drop between inlet and outlet levels. By my method of thinpulp grinding in a mill of roughly equal diameter and length, with all conditions set as prescribed to produce optimum conditions for classification in the pool and different optimum conditions for grinding in the ball mass, an entirely new and revolutionary method of fine comminution is made available which reduces power consumption in making a ground product of a set maximum size, Which effects a new and improved grain-size distribution in the product, and which in the preparation of feeds for sulfide flotation and the like, wherein harmful chemical time-reactions occur, reduces the overall time-factor by a large fraction and, as in the case of sulfide flotation, reduces also the access per unit time of the harmful reactant, oxygen.

Figure 1 illustrates one form of conventional ball mill in longitudinal cross section and Figure 2 is a sketch showing the position of the ball mass and the pool maintained within such a mill during operation in accordance with the invention. The mill illustrated, with the dimensions indicated, is that which was used in the runs subsequently described, its construction being conventional except that it was operated with a large discharge opening (33 inches) instead of the more or less conventional 12-inch discharge opening.

In operation the ball mass assumed the position shown when a ball volume of about 45 percent (1 inch forged steel balls) was used with a rotation speed of 13.7 R. P. M. The feed was a 37.6 percent by weight pulp of a massive copper sulfide ore which had been ground in a rod mill and had the following size range:

Table] Percent +20 mesh 5.9 +28 mesh 8.2 +35 mesh 13.2 +48 mesh 13.3 +65 mesh 11.9 +100 mesh 10.6 +150 mesh 8.2 +200 mesh 6.3 200 mesh 22.4

It was" fed to the ball mill at the rate of 2100 tons of ore per day, and the ball-mill effluent, without further classification, was fed to a flotation unit. The size dis- In contrast with this, the same mill operated conven tionally, with hydroscillator for external classification, with return of oversize to the mill, operating at the same tonnage on a feed of the same ore having the size distribution:

Table III I Percent +20 mesh 6.1 +28 mesh 8.4 +35 mesh 12.6 +48 mesh 13.7 +65 mesh 12,3 mesh 10.1 mesh 8.4 +200 mesh 5.9 200 mesh 22.5

gave a product having the following size distribution:

Table IV Percent +20 mesh +28 mesh +35 mesh +48 mesh 1.2 +65 mesh 3.6 +100 mesh 6.7 +150 mesh 11.9 +200 mesh 18.8 +325 mes'h 27.3 +800 mic 20.2 +1600 mic 5.2 1600 mic 5.1

In this operation, the pulp density of the feed to the mill was 72.9%, which had been determined by considerable investigation to be the optimum for this set-up. The speed, ball load and ball size were the same in both runs.

It will be noted that operation in accordance with the invention resulted in a considerable reduction in the amount of 200-mesh size produced and an even greater increase in the amount of 325-mesh sands produced.

Flotation results on the products showed 0.09% copper and 6.7% of sulfur in the tailings from the product ground in accordance with the present invention, and 0.13% copper and 6.4% sulfur in the taili-ngs from the products processed conventionally, indicating a reduction in loss of nearly 31% in the product ground in accordance with the present invention. Table V below shows the weight and copper distributions in the tailings made during companable seven-day periods of operation with inmill classification (columns 2 to 4) and conventional closed-circuit flow (columns 5 to 7). Column 8 shows the striking improvement in the copper removal effected at the different sizes by operation according to my invention. The decreases indicated in the two coarsest sizes are, I found, owing to a drip which was observed coming from the inner edge of the discharge trunnion during both operations and which was caused by prominent ra'dial ribs on the discharge-end liner used in both. In the operation of the invention this drip fell into the finished product and contaminated it whereas in the conventional operation the drip fell into the feed to the external classifier and there settled into the return sands. Elimination of dn'p or similar return may be effected in the operation of my invention by using less prominent ribbing or by use of the drip trough described below.

I attribute the superior showing of the operation according to my invention to reduction in the amount of preferential grinding of sulfides that normally occurs in conventional circuits. This is indicated in the data by the fact that the flotation treatment in both cases shown was precisely the same, wherefore equal recovery per- 75 formance of the flotation machines on the feeds sent them in the two runs must be postulated; On the further postulate of any given recovery x in the 1600 range in the two cases the assays of the 1600-mesh size in the feeds would have been 0.21/x and 0.43/x for the new operation and the conventional operation respectively, wherefore less than half as much copper was thrown into the 1600 slimes in the new operation as in the conventional. Such a conclusion is substantiated by consideration of the classifying mechanism in my operation. Thus upward feeding through the pool bottoms imparts an upward momentum to the sulfide sands and since they are smaller than their equal-settling gangue counterparts they can pass upwardly through the interstices of the teeter column with less interference than equal-settling gangue. Further, the teeter column in the in-rnill classifier is completely undisturbed by any penetrating machine parts in contradistinction to the rake supports in conventional reciprocating rake classifiers or the revolving ribbon in spiral classifiers both of which produce channels through which tramp material descends. Again the rising water currents in the in-mill classifier are not subjected to the relatively violent surging and reversals caused by reciprocating rakes nor the reversing strong swirls in the hydroscillator bowl. And finally, in in-mill classification, sands that reach the upper layer are skimmed 011 much more rapidly than in conventional classifiers because of the restricted overflow and the more extensive reach of accelerated horizontal surface flow induced thereby.

minor structural changes and additions to the mill as noted below. Thus, the opening in the discharge trunnion should be relatively large to increase the. velocity of flow of the surface layer of pulp flowing from the classifying pool through the opening in the discharge trunnion. The discharge-end liner should be so arranged as to prevent the ribs (if they are ribbed) from acting as sand elevators. This can be accomplished by installing a trough to catch drippings from the top of the shell and return them to a point near the feed end, so that they do not drip into the classifying pool directly at the outlet weir and so introduce tramp oversize. If necessary water may be flowed into the trough to carry such pulp as drops into it back to the feed end. In view of the fact that the discharge trunnion has a large opening, support for such a trough is easily arranged. Use of a splash board at the outlet has already been mentioned.

I claim:

In a process of simultaneously wetagrinding and classitying a sulfide ore in which ore of a mesh not exceeding about IO-mesh is fed to a horizontal axis, substantially cylindrical ball mill having a diameter approximating its length, having a discharge opening of size sufiicient to provide hall-unimpeded discharge of pulp and having a load of small balls, the mill being rotated at below critical speed whereby a classifying pool of pulp undisturbed by the free fall of cataracting balls thereinto and having a depth of at least 10 inches over the toe of Table V Tailing with in-mill classi- Tailing with conventional fication external classification Percent 1m- Mesh Size provement 1n copper Percent Percent Percent Percent Percent Percent removal weight Cu S weight Cu S 48 3. 7 0. 23 7. 2 3.0 0.08 0. 8 Decrease. 65 7. 8 0.21 7. 6 8.7 0. 12 0. 9 D0. 100 14. 8 0. 14 8. 5 14. 8 0. 17 2. 2 17-7- 150 15.7 0.07 5. 9 16. 5 0.15 9. 6 53.1. 200. 12. 8 0.05 4.1 14. 7 0.07 6. 6 28.6. 325 20. 2 0. 04 3.1 16. 3 0. 06 3. 7 33.3. 800 microns 15. 6 0.04 7. 6 13. 5 0.06 5. 3 33.3. 1,600 microns. 3. 4 0.09 14. O 5. 7 0. 14 12. 2 35.7. 1,600 microns 6.0 0.21 14. 6 6.8 0.43 13.0 51.0. Composite 100.0 0. 09 6. 7 100.0 0. 13 6. 4 30.7.

In the operation carried out in accordance with the invention described above, the mean horizontal rate of fiow of the surface pulp in the classifying pool was about 21 feet per minute, as measured by a wooden float dropped into the feed scoop.

I have pointed out above that the design of the ball mill which is used in the practice of the invention is substantially conventional, and that the invention lies in utilization of the space in the mill unoccupied by the ball mass to form a new and highly efficient classifier. This is brought about by control of the pulp density, ball charge, ball size, and speed of rotation with certain References Cited in the file of this patent UNITED STATES PATENTS Hutchins Feb. 21, 1933 Hall May 28, 1935 

